Method of assessing solderability

ABSTRACT

A sequential electrochemical reduction method and apparatus for assessing solderability of electronic component leads and printed wiring boards. The method detects and quantifies the oxides present on copper, solder, and intermetallics that are detrimental to solderability. A solderable portion of the component to be tested is immersed in an electrolyte to form an electrode. An inert counter electrode and a reference electrode are also placed in the electrolyte. A current is passed from the inert counter electrode to the tested component, and the potential between the component and the reference electrode is recorded as a function of time. In a plot of the electrode potential versus the total charge passed, a series of inflection points identify and quantify particular metallic oxides present on the solder. The plot is compared with previous analyses of aged specimens having known oxide compositions that correlate with degradation of solderability. The method is useful for testing off-the-shelf components and for control of circuit board manufacturing and assembly processes.

TECHNICAL FIELD

The present invention relates to methods of testing electroniccomponents and, in particular, to a method and apparatus for assessingthe solderability of electronic component leads and printed wiringboards.

BACKGROUND OF THE INVENTION

A major cost problem experienced by the electronics industry is the lossof solderability of electronic components and printed circuit boards,particularly during storage. Poor solderability of component leads andprinted wiring boards is believed to account for as much as 75% ofsolder joint failures. Because humid environments are known toexacerbate the problem, an electrochemical mechanism is clearly thecause of solderability degradation. In the lead-tin-copper soldersystem, for example, previous studies have determined that oxidation ofthe tin-lead (Sn-Pb) surface and underlying copper-tin (Cu-Sn)intermetallic layers is involved in the degradation process. In thepast, however, the nature of the various oxides and their roles in thedegradation of solderability remained obscure.

Traditional techniques typically employed in the prior art for surfaceanalysis of circuit boards provide only subjective indicators ofsolderability. Currently used production test methods are alsodestructive by nature. Because degradation of solderability is known toinvolve an electrochemical mechanism, it is believed that solderabilitycan be assessed more accurately and efficiently using electrochemicalmethods that provide in situ quantitative analysis of metallic oxidesknown to degrade solderability. In particular, there is a need forquantitative, nondestructive, electrochemical methods of solderabilityanalysis that are easily applied for testing off-the-shelf componentsand for process control in the production environment.

SUMMARY OF THE INVENTION

The present invention comprises a sequential electrochemical reductionmethod and apparatus for assessing solderability of electronic componentleads and printed wiring boards. The process is applicable to thelead-tin-copper solder system, as well as to solder systems comprisingother metals and alloys. The method detects and quantifies metallicoxides known to degrade solderability when present on the solder surfaceand any intermetallic layers. The invention comprises a nondestructivemethod that provides a quantitative measure of the solderability ofelectronic components and printed wiring boards.

The electrochemical reduction method of the present invention isperformed by placing the solderable portion of the component or circuitboard to be tested in an electrolyte, such as a borate buffer solution.The immersed component forms a first electrode. A second, inertelectrode and a third, reference electrode, such as a saturated calomelelectrode, are also placed in the electrolyte. A small cathodic currentis passed from the inert electrode to the tested component, and thepotential between the component and the reference electrode is recordedas a function of time. In systems where the second, inert electrode hasa stable voltage at the low currents used, it can also function as thereference electrode, thereby eliminating the need for a separatereference electrode. In a plot of the electrode potential versus thetotal charge passed (current multiplied by time), a series of inflectionpoints or plateaux are observed in which the voltage level identifies aparticular oxide or oxide mixture, and the associated charge is ameasure of the thickness of that particular oxide.

The results achieved from the sequential electrochemical reductionperformed on the tested component are compared to similar analyticalresults from baseline experiments on specimens exposed to various agingtreatments. As determined by a wetting balance method, solderability ofaged specimens has been found to correlate with results of thesequential electrochemical reduction method of analysis. Therefore, theanalytical results from the tested component can be compared withbaseline results obtained from aged specimens having known oxide andsolderability characteristics to determine the solderability of thetested component.

A principal object of the invention is to improve solderability ofelectronic components and circuit boards by identifying and minimizingthe types and amounts of metallic oxides that are present on the soldersurfaces and intermetallic layers. A feature of the invention is the useof a sequential electrochemical reduction method to identify themetallic oxides that cause loss of solderability in electroniccomponents. An advantage of the invention is that it provides anondestructive method that is easy to perform and that yields aquantitative measure of the solderability of electronic components. Thisinformation is useful for controlling production soldering processes andfor improving the manufacturing processes for printed wiring boards andother electronic components.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther advantages thereof, the following Detailed Description of thePreferred Embodiments makes reference to the accompanying Drawings, inwhich:

FIG. 1 is a graph of electrode potential versus charge densityillustrating the sequential electrochemical reduction of the oxides ofSn, Pb, and eutectic Sn-Pb, and indicating the voltage ranges over whichthe various oxides are reduced;

FIG. 2 is a table showing a correlation between the presence of higherSn oxides and poor solderability, as indicated by the long wetting timesmeasured using a wetting balance method;

FIG. 3 is a schematic diagram of an apparatus for assessing thesolderability of a printed wiring board through-hole electrode; and

FIG. 4 is a schematic diagram of an apparatus for assessing thesolderability of a printed wiring board surface-mount pad electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Solder coatings are widely used to protect copper in printed wiringboards and electronic component leads from oxidation that can lead toloss of solderability. A typical solder coating comprises eutecticSn-Pb, for example, that can be applied directly by hot dipping or canbe electroplated and then densified by reflowing (melting).Solderability of a finished component is affected by the metal ratios ofthe deposited solder, the thickness of the coating, the type of solderbath, the plating conditions, the presence of organic contaminants fromplating bath additives, and the generation of breakdown products. Whensufficiently thick and properly applied, such coatings can retain theiroriginal solderability even after several years of normal storage.However, Sn-Pb coatings frequently oxidize and lose solderability as aresult of poor coating quality and/or a poor storage environment.

BASELINE EXPERIMENTS

A typical test specimen was a 1.5 mm diameter hard Cu wire, 2.5 cm inlength, which was masked with Teflon® heat-shrink tubing to expose a 1cm long section with a rounded end. The exposed section was plated with10 μm of Cu from a standard non-additive pyrophosphate bath a 55° C.,then with 12 μm of eutectic Sn-Pb from a standard fluoroborate bath atroom temperature. During plating the wire cathode was rotated at 2000rpm to control mass transport in the solution. A 60/40 Sn-Pb ratio wasverified by atomic absorption analysis of specimens dissolved in acidsolution and by X-ray fluorescence analysis. The Sn-Pb coating wasreflowed in water soluble oil at 235° C. for minimal time prior to use.Specimens were subjected to various steam aging and anodizationtreatments to produce oxidized samples.

Electrochemical reduction was performed on the specimens in a boratebuffer solution (9.55 g/L sodium borate and 6.18 g/L boric acid) at a pHof 8.4 under an argon atmosphere in a 200 mL glass cell having separatecompartments for a Pt counter electrode and a reference saturatedcalomel electrode (SCE). Other electrolytes compatible with the Cu-Sn-Pbsystem may be used in conjunction with alternative types of referenceelectrodes and counter electrodes of other inert materials. Allelectrochemical experiments were performed using apotentiostatgalvanostat (PAR model 173, EG&G Princeton Applied ResearchCorp., Princeton, N.J.). Solderability tests were performed using amodified Wilhelmy wetting balance in conjunction with a digitaloscilloscope (Nicolet model 2090, Nicolet Instrument Corp., Madison,Wis.).

A constant reduction current of -20 μA/cm² was applied to the specimenelectrode in the borate buffer solution and the electrode potentialversus the reference SCE electrode was recorded as a function of time.Inflection points, which are believed to correspond to the reduction ofPbO, SnO, and SnO₂, were observed at approximately -0.6 V, -0.9 V, and-1.1 V, respectively. From cyclic voltammetric measurements involvingpure Pb and Sn electrodes, it was determined that there was a one-to-onecorrespondence between the anodic charge passed during formation of theoxides by anodization and the cathodic charge required for theirreduction. This result indicates that the electrochemical reductionmethod of the present method provides a quantitative measure of theamount of each surface oxide present. In addition, the fact that suchthick oxides can be sequentially reduced indicates appreciable oxideporosity. Heavily oxidized specimens were consistently found to exhibitvery poor solder wetting characteristics, thereby establishing a linkbetween the presence of higher Sn oxides and the loss of solderability.

FIG. 1 illustrates representative curves obtained from sequentialelectrochemical reduction analysis of the oxides formed on Sn, Pb, andeutectic Sn-Pb under mildly oxidizing conditions. These curves plotelectrode potential versus charge density (which equals current density×time). The curves comprise a series of inflection points or plateaux inwhich the voltage level identifies a particular oxide, and theassociated charge is a measure of the oxide thickness. In Sn-Pbcoatings, the two Pb oxides normally are present only in smallconcentrations. It is convenient to refer to the Sn oxides as the loweroxide (believed to be predominantly SnO), which reduces at a voltage ofapproximately -0.8 V to -1.0 V, and the higher oxide (believed to bepredominantly SnO₂), which reduces at a voltage of approximately -1.0 Vto -1.4 V (versus the reference SCE electrode). Negative voltage peaksare believed to indicate a duplex structure having an outer layer thatis more difficult to reduce than the underlying material.

FIG. 2 is a table of data obtained from testing Sn-Pb solder specimensas-reflowed (zero aging) and after steam aging (24 hours). The chargedensity for reduction (measured in mC/cm²) is an indication of theamount and type of oxides present. The wetting time is a measure of thetime in seconds to obtain two-thirds of the maximum theoretical wettingforce obtainable by a wetting balance method. The charge density dataand the wetting time data are reported for similarly prepared specimens,but not the identical specimens. It should also be noted that because ofthe high thermal inertia of the relatively thick test specimens used inthis study, wetting times are generally higher than would be expectedfor thinner specimens, but it is the relative values that are important.From the data in FIG. 2 it can be seen that the presence of largeamounts of higher Sn oxide detected by the electrochemical reductionmethod of the present invention correlates with a loss in solderability,as indicated by the long wetting times. The present method has also beenuseful in detecting more subtle losses in solderability than thosepresented FIG. 2.

The copper oxides, Cu₂ O and CuO, and the Cu-Sn intermetallic oxides canalso be detected readily by the sequential electrochemical reductionmethod of the present invention. The principal Cu-Sn intermetallic oxidewas investigated by plating standard Cu-plated wire specimens with 3 μmof Sn and heating under a vacuum at 200° C. for three days to convertall of the Sn to Cu₃ Sn. These specimens were found to be almostcompletely unwettable by solder. A layer of native oxide, about 5 atomlayers thick, was found to be completely reduced in the argon-saturatedborate buffer electrolyte. The reduced surface was stable in thedeaerated solution, but exposure of the reduced surface to air for only5 seconds resulted in regrowth of 5 atom layers of oxide that exhibiteda well-defined plateau at about the same voltage as observed for SnOreduction (-0.9 V). Overnight, this oxide thickened to greater than 10atom layers, and the reduction curve exhibited a long tail, indicatingthe presence of the more stable Sn oxide. These results show that amajor problem with the Cu₃ Sn intermetallic is the fast rate at whichrelatively thick oxide layers form on its surface. It is believed thatthis is a consequence of local cell action in which Cu acts as thecathode.

ASSESSING SOLDERABILITY

The method of assessing solderability of an electronic component isessentially the same as described above in the baseline experiments. Anelectronic component lead to be tested is immersed in an electrolyte toform a first electrode. A second, inert counter electrode and a third,reference electrode are also placed in the electrolyte. A negativecurrent is passed between the component and the second electrode whilethe potential between the component and the reference electrode isrecorded as a function of time. As described above, in systems where thesecond, inert electrode has a stable voltage at the low currents used,it can also function as the reference electrode, thereby eliminating theneed for a separate reference electrode. The voltage versus chargedensity curve obtained indicates the various types and amounts of oxideson the component lead. This data is compared with known results from thebaseline experiments to characterize the solderability of the componentlead.

Assessing solderability of printed wiring boards can be accomplishedwith apparatus similar to those depicted schematically in FIGS. 3 and 4.The apparatus of FIG. 3 is suitable for testing through-hole solderpoints and the apparatus of FIG. 4 is suitable for testing solder padsused for surface-mount components. The same reference numerals are usedin FIGS. 3 and 4 to identify the same or similar elements of the twodevices. In FIG. 3, a wiring board 20 having bare or solder-coatedprinted copper circuitry to be tested for solderability is clampedbetween an upper jaw 22 and a lower jaw 32. Upper jaw 22 has an aperture24 and an O-ring 26. O-ring 26 is attached to the lower side of jaw 22and encircles aperture 24. Lower jaw 32 has an aperture 34 and an O-ring36. O-ring 36 is attached to the upper side of jaw 32 and encirclesaperture 34. A chamber 40 having a centering pin 41 is mounted atop jaw22. Chamber 40 is mounted on jaw 22 so that pin 41 can extend downwardthrough aperture 24 in jaw 22. Pin 41 may be used to position athrough-hole electrode 46 of printed wiring board 20 between O-rings 26and 36. When circuit board 20 is positioned by pin 41 and clampedbetween jaws 22 and 32, O-rings 26 and 36 provide a seal aroundthrough-hole electrode 46 in preparation for assessing solderability ofthe bare or solder-coated printed copper circuitry associated withthrough-hole 46.

A tube 48 of inert material extends from aperture 34 of lower jaw 32into a sealed electrolyte reservoir 50. Reservoir 50 contains anelectrolyte solution 52 and an inert gas 54, such as argon, aboveelectrolyte 52. Electrolyte 52 may comprise any electrolyte compatiblewith the particular solder system, such as a borate buffer solution(9.55 g/L sodium borate and 6.18 g/L boric acid at a pH of 8.4, forexample) suitable for use with the Cu-Sn-Pb system. A wide variety ofelectrolytes (e.g., borates, citrates, sulfates, nitrates, etc.) willprovide acceptable results. However, electrolytes having a neutral oralkaline pH, and from which strong metal complexing agents (e.g.,chloride, bromide, etc.) have been excluded, will yield the mostaccurate measurements. Inert gas 54 is supplied to reservoir 50 througha gas line 56 from a gas source 58. Gas 54 can exit reservoir 50 througha gas outlet valve 57. Inert gas 54, such as argon, is used to flush airfrom the system to eliminate erroneous electrochemical reduction datacaused by the presence of oxygen. A reference electrode 60, which maycomprise a saturated calomel electrode (SCE), for example, extends intoreservoir 50 and into electrolyte 52. Reference electrode 60 may beplaced in a separate compartment within reservoir 50 to minimize theeffects of any contamination of the electrolyte.

A control system 62, comprising a current source, a voltage meter, and arecording device, is connected by leads 61, 63, and 65, respectively, toreference electrode 60, an inert counter electrode 64 (such as platinum,for example) that extends within the chamber of chamber 40, and athrough-hole contact 66 that is connected by the printed circuitry ofcircuit board 20 to through-hole electrode 46. In an alternativeembodiment, if inert electrode 64 has a stable voltage at the lowcurrents used, it can also function as the reference electrode, therebyeliminating the need for separate reference electrode 60.

Through-hole electrode 46 is tested for solderability by using pin 41 toposition through-hole 46 of circuit board 20 between O-rings 26 and 36.Circuit board 20 is clamped securely by jaws 22 and 32 so that O-rings26 and 36 form a seal around electrode 46. Pin 41 may then be withdrawnfrom through-hole 46. Chamber 40 is connected to a vacuum line 42controlled by a valve 43 and to an inert gas line 44 controlled by avalve 45. Valve 45 is opened to flush chamber 40, through-hole 46, andtube 48 with inert gas to remove oxygen from the system. Thereafter,valve 45 is closed and valve 43 is opened so that electrolyte 52 isdrawn up through tube 48, through and around electrode 46, and intochamber 40 above counter electrode 64.

With electrolyte 52 drawn into chamber 40, system 62 provides a constantcurrent, in the range of about 10-1000 μA/cm², through line 63,electrode 64, electrolyte 52, electrode 46, through-hole 66, and line 65back to source 62 (i.e., a negative current is supplied fromthrough-hole electrode 46 of circuit board 20 to counter electrode 64).The current provided by source 62 causes sequential electrochemicalreduction of the oxides on the bare or solder-coated copper circuitry ofthrough-hole electrode 46. Current greater or less than the recommended10-1000 μA/cm² may be used: a low current provides high resolution atthe expense of time; a high current provides fast results but lowresolution. While supplying current, system 62 measures and records theelectrode potential between through-hole electrode 46 and referenceelectrode 60 as a function of time. The time factor can be converted tocharge density by multiplying the current density by the elapsed time.As described above, the readout of electrode potential versus chargedensity (or time) produces a series of inflection points or plateau thatindicate the particular oxides being reduced as well as the thicknessesof the various oxide layers. The results can be compared to the baselinedata to determine the specific oxides present on electrode 46 and theassociated measure of solderability. Although a typical circuit board 20comprises a multiplicity of through-hole electrodes, a small number ofthrough-holes (a statistical sample) can be tested to characterize thesolderability of the entire circuit board 20.

The apparatus shown schematically in FIG. 4 is simply a modification ofthe foregoing system suitable for testing a solder pad electrode 68 oncircuit board 20 instead of a through-hole. The modified apparatuscomprises an open-bottom vessel 70 that has an O-ring 20 around itsbottom rim. Vessel 70 is placed atop board 20 so that O-ring 72surrounds solder pad electrode 68. Vessel 70 typically includes a lid orport (not shown) for adding electrolyte 52. A clip (not shown) can beused to secure board 20 to the bottom of vessel 70 so that a tight sealis maintained by O-ring 72 around solder pad electrode 68. Inert gas 54is supplied to vessel 70 by line 56 from gas source 58. After a seal ismade around pad electrode 68, vessel 70 may be flushed with inert gas54, with air escaping from vent 57. Vessel 70 may include a connectedchamber 74 with a porous glass frit 75 for partially isolating referenceelectrode 60. System 62 provides current for electrochemical reductionof the metallic oxides on solder pad electrode 68. Current is providedfrom control system 62 through line 63, electrode 64, electrolyte 52,pad electrode 68, circuit-connected pad electrode 69, and line 65 backto system 62. System 62 also measures and records the electrodepotential between pad electrode 68 and reference electrode 60 as afunction of time during electrochemical reduction of the metallic oxideson pad electrode 68. Electrode 60 may be placed in chamber 74, asillustrated, or electrode 64 may function as the reference electrode insome circumstances, thereby eliminating the need for a separatereference electrode 60. As described above, the record of electrodepotential versus time can be compared to the baseline data to determinethe specific oxides present on pad electrode 68 and the associatedmeasure of solderability.

Although the present invention has been described with respect tospecific embodiments thereof, various changes and modifications can becarried out by those skilled in the art without departing from the scopeof the invention. Therefore, it is intended that the present inventionencompass such changes and modifications as fall within the scope of theappended claims.

We claim:
 1. A method of assessing solderability of an electroniccomponent, comprising the steps of:placing a solderable portion of thecomponent in contact with an electrolyte to form a first electrode;placing a second electrode in contact with said electrolyte; connectingsaid first and second electrodes to a source of electric power andpassing a current between said first and second electrodes; reducingmetallic oxides present on said solderable portion of the component;measuring voltage and current between said first and second electrodesas a function of time during said reduction of metallic oxides; andanalyzing said voltage and current versus time measurements to determinesolderability of the component.
 2. The method of claim 1, wherein thestep of analyzing further comprises:correlating said voltage and currentversus time measurements with predetermined baseline data from testspecimens having known solderability.
 3. The method of claim 1, furthercomprising the steps of:providing a sealable reservoir for containingsaid electrolyte; and flushing said electrolyte with an inert gas toeliminate oxygen from said electrolyte.
 4. A method of assessingsolderability of an electronic component, comprising the stepsof:placing a solderable portion of the component in contact with anelectrolyte to form a first electrode; placing a second electrode incontact with said electrolyte; providing a third, reference electrode incontact with said electrolyte; connecting said first and secondelectrodes to a source of electric power and passing a current betweensaid first and second electrodes; reducing metallic oxides present onsaid solderable portion of the component; measuring current between saidfirst and second electrodes and voltage between said first and thirdelectrodes as a function of time during said reduction of metallicoxides; and analyzing said voltage and current versus time measurementsto determine solderability of the component.
 5. A method of sequentialelectrochemical reduction of metallic oxides for assessing solderabilityof an electronic component, comprising the steps of:providing anelectrolyte in a reservoir; placing a solder coated portion of thecomponent in contact with said electrolyte to form a first electrode;placing a second, inert electrode in contact with said electrolyte;connecting said first and second electrodes to a source of electricpower; sequentially reducing metallic oxides on said solder coatedportion of the component by passing a current between said first andsecond electrodes; measuring voltage and current between said first andsecond electrodes as a function of time during said sequential reductionof metallic oxides; and analyzing said voltage and current versus timemeasurements to determine solderability of the component.
 6. The methodof claim 5, wherein the step of analyzing further comprises:correlatingsaid voltage and current versus time measurements with predeterminedbaseline data from test specimens having known solderability.
 7. Amethod of sequential electrochemical reduction of metallic oxides forassessing solderability of an electronic component, comprising the stepsof:providing an electrolyte in a reservoir; placing a solder coatedportion of the component in contact with said electrolyte to form afirst electrode; placing a second, inert electrode in contact with saidelectrolyte; placing a third, reference electrode in contact with saidelectrolyte; connecting said first and second electrodes to a source ofelectric power; sequentially reducing metallic oxides on said soldercoated portion of the component by passing a current between said firstand second electrodes; measuring current between said first and secondelectrodes and voltage between said first and third electrodes as afunction of time during said sequential reduction of metallic oxides;and analyzing said voltage and current versus time measurements todetermine solderability of the component.
 8. The method of claim 7,further comprising the steps of:eliminating oxygen from said electrolytein said reservoir, said reservoir comprising a sealable reservoir; andplacing a saturated calomel electrode as said third reference electrodein a compartment of said reservoir.